Can we prove that God exists simply by thinking of God? In other words, is it possible to prove that God must exist on the basis that you have an idea of God in your mind? According to the philosopher and theologian St Anselm, it is indeed possible.

The ontological argument is different from the previous two arguments we have considered in that is an a priori argument. The cosmological argument and the teleological argument, you may recall, are a posteriori. A posteriori knowledge is the most common form of knowledge we possess. This form of knowledge is based on experience. That is to say, we acquire it through observation, looking up the information in a reliable reference book, past experience and so on. Two examples are my knowledge that Hitler was appointed Chancellor in 1933, or my knowledge that sunflowers have yellow petals.

However, it may be argued that not all of our knowledge comes from experience alone. Take, for example, the fact that 2 + 2 = 4. Such mathematical formulations, some would argue, can be determined prior to experience; provided, of course, you are already familiar with the number system. Anselm presents the view that the same could be said for the existence of God.